1. Field of the Invention
This invention relates in general to power tongs used for rotating casing, tubing and pipe in earth drilling and reaming operations and the like and, in particular, to power tongs with movable jaws that grip pipe to make-up and break-out threaded connections.
2. Description of the Prior Art
Power tongs associated with well drilling work are well known in the art. Such prior tongs have generally been classified as either open head or closed head tongs. A lateral passageway is provided in the open head tong which allows the pipe to be engaged or disengaged by moving the tong laterally with respect to the vertical pipe axis. The closed head tong, being formed in the shape of a closed circle, can only be installed on the pipe by lowering the tong over the pipe while the pipe is held by slips at the rig floor. The power tong shown in U.S. Pat. No. 1,811,666 to W. W. Foster, issued June 23, 1931, entitled "Automatic Power Driven Pipe Wrench" is a closed head tong. The closed head tong remains around the pipe for subsequent make-up and break-out operations and must be stripped over the pipe for removal.
In addition to the open head or closed head terminology, power tongs are also generally classified as unidirectional or bidirectional in operation depending on the jaw system employed. A unidirectional tong has jaws which close and grip pipe when the tong rotates in a single direction. If the tong is rotated in the opposite direction, the jaws are disengaged and retracted from the pipe gripping position. Bidirectional tongs have jaw systems which permit left hand or right hand rotation of the pipe being gripped, depending on the direction of rotation of the tong. Open head tongs exist which are bidirectional in operation, in some cases without reversing the jaw elements by hand or inverting the tong unit.
The power tong shown in U.S. Pat. No. 4,060,014 to John W. Turner, Jr., issued Nov. 29, 1977, entitled "Power Tong" is an open head tong which is bidirectional in operation without inverting the unit or manually reversing the jaw elements. Turner's device can also accommodate greater pipe diameters than did prior open head units. In spite of these advantages, Turner's device is shiftable between the make-up and break-out modes only when a pipe is not located within the tong head. Changing modes also requires the removal and reinsertion of a pin between holes in the tong head.
There exists a need for a power tong which will allow the direction of jaw rotation to be reversed while the tong is located on the pipe and which will allow the jaws to be moved to a fully retracted position within the tong head to allow pipe to be tripped through the head.